FIELD OF THE INVENTION
This invention relates to optical communication systems.
One object of the invention is to provide a means establishing and maintaining communication between two or more stations whose relative location is initially unknown, and establishing such communication very rapidly.
According to the present invention, an optical communication system comprises a transmitter including means for producing a frequency-modulated optical beam, acousto-optic deflector cell means for scanning said beam and for introducing a frequency shift into the frequency-modulated optical beam; and a receiver including means for detecting a frequency-shifted, frequency-modulated optical beam and means for determining the frequency thereof.
The means for producing a frequency-modulated optical beam may, conveniently, be a laser coupled to an acousto-optic modulator.
The means for determining the frequency of a frequency-shifted, frequency-modulated optical beam may comprise a coherent detection system employing heterodyne techniques, for example.
The deflector cell means may comprise any suitable material which exhibits an acousto-optic effect ie. diffraction of light by acoustic waves.
One example of such a material is tellurium dioxide. Acoustic waves are usually coupled into the cell via a piezoelectric transducer, for example, which is bonded to one face of the cell.
The theory of operation of acousto-optic devices is well-documented- See for example "Principles of Acousto-Optic Devices" by V. M. Ristic.
As is known, an acousto-optic deflector cell may be configured to receive a beam of laser light, of frequency f.sub.o say, and in response to a high frequency drive signal applied to the cell (in the MHz to GHz range), some of the light emerging from the cell is deflected to form a so-called "first order beam". The angle of deflection of this beam with respect to the undeflected zero order beam is substantially proportional to the frequency of the drive signal, (fac) which initiates a sound wave which propagates across the cell. Hence by varying the drive signal frequency in some controllable manner, a beam may be scanned in a single plane. Two dimensional scanning may be achieved by incorporating a second deflector cell through which the first order beam. produced by a first cell is arranged to pass. Various means for producing a two-dimensionally scanned beam (for missile guidance applications) are described in GB-A-Z, 113,939 for example.
In addition to suffering a deflection, the first order beam also undergoes a frequency shift, such that the frequency f.sub.d of the deflected, first order beam can be written; EQU f.sub.d =f.sub.o .+-.fac
i.e. the frequency of the deflected beam is shifted with respect to that of the incident light by an amount equal to the frequency of the drive signal. In effect, the incident light is Doppler-shifted by the frequency of the acoustic waves which propagate across the cell. The direction of the shift (i.e. an increase or decrease in frequency) is determined by the direction of propagation of the sound wave relative to the incident laser radiation.
It follows then that the angle of deflection is uniquely related to this "Doppler" shift and thus that every beam angle emerging from the cell is uniquely coded by means of a unique frequency shift. The present invention exploits this Doppler shift effect.